I've felt like that many, MANY times. I'm not on antidepressants (at least not yet), so the only thing I find that helps is to just ride it out. Once you get to your lowest point, you can only go back up. I've come to the brink of taking my own life a few times, but I've always come back to the idea that there is a purpose for me, even if I feel completely useless and alone. If I died, it would devestate my family, my boyfriend, and my best friends, and that's enough to make me keep pushing forward. I couldn't do that to them, even for my own sake.
The important thing when you feel like this is to get up and do something. Anything. Watch a movie, play a game, something to take your mind off of the mess in your head. Can't do it at home? Go out. Force yourself to get up and go somewhere. The mall, the movies, whatever. Go out with a friend for some coffee, and talk about how you feel to someone you trust not to jump you and put you in a mental ward. My boyfriend has been subjected to many of my suicidal thoughts, but each time he doesn't judge me. He just gives me reasons not to give up. Basically, he picks me up and dusts me off and sends me on my way again. Doesn't have to be a boyfriend, either. It can be your best friend, or even a sibling (if you trust them not to freak out).
Just try not to think about it too much. Distract yourself, and eventually, you'll get to being your old self again. <img src="i/expressions/face-icon-small-smile.gif" border="0">